A shock absorber for damping vibration of a vehicle, equipment, a structure and the like is known. The shock absorber is used for a saddle-type vehicle such as a bicycle, a tricycle and the like, for example. Such a shock absorber is composed of an outer tube and an inner tube going into and out of this outer tube. A telescopic shock absorber body which becomes an outer shell of the shock absorber is provided, and the shock absorber body accommodates a suspension spring for elastically supporting a vehicle body. Such a shock absorber generates a damping force for suppressing a telescopic motion of the shock absorber body.
In some cases, such a shock absorber is provided with a cylinder a rod guide, a piston rod, a piston, a piston passage, a damping valve, a free piston, urging means, and a relief portion which will be explained below.
The cylinder stands in a shaft center portion of the shock absorber body. In the cylinder, a liquid chamber filled with an operating liquid therein is formed. The rod guide has an annular shape, fixed to the cylinder and closes one side of the liquid chamber. The piston rod penetrates the rod guide and goes into and out of the cylinder with extension and contraction of the shock absorber body. The piston is held at a distal end portion of the piston rod and divides the liquid chamber into a rod-side chamber and a piston-side chamber. The piston passage is formed in the piston and has the rod-side chamber and the piston-side chamber communicate with each other. The damping valve applies resistance to the operating liquid passing through the piston passage. The free piston is in sliding contact with a portion on a side opposite to the piston rod side in an inner peripheral surface of the cylinder and closes the other side of the liquid chamber. The urging means urges the free piston to the liquid chamber side. The relief portion allows the operating liquid in the cylinder to escape to an outside of the cylinder when the free piston retreats by a predetermined amount against an urging force of the urging means. That is, it relieves the operating liquid.
In the above-described shock absorber, at the time of extension and contraction of the shock absorber when the piston rod goes into and out of the cylinder, the operating liquid in one of the chambers pressurized by the piston passes through the piston passage and moves to the other chamber. As a result, a damping force caused by resistance of the damping valve is generated. Moreover, in the above-described shock absorber, the urging means pressurizes the liquid chamber. As a result, in the above-described shock absorber, responsiveness of generation of the damping force is made favorable. In the above-described shock absorber, even if the operating liquid in the cylinder increases or swells, the relief portion suppresses an excessive rise of a cylinder internal pressure. Such a shock absorber is disclosed in JP2005-30534A, JP2010-270832A, and JP2011-117533A, for example.